1. Field of the Invention
The invention relates to particles less than 100 .mu.m in size, which are insoluble in organic solvents and water and modified at their surface by hydrophilic and hydrophobic groups. It also relates to processes for modifying these particles as well as to the use of the modified particles as surface active products, especially for stabilizing or destabilizing emulsions and foams and for tertiary oil recovery.
2. Description of the Prior Art
The hydrophilic or hydrophobic behavior of a solid is determined essentially by the groups at its surface. For example, particles which have hydroxyl groups at their surface are usually hydrophilic. Such behavior is shown, for example, in finely divided pyrolytic silica. However, if there are hydrocarbon groups, such as, alkyl groups, and especially longer chain alkyl groups, at the surface of the particle, then the particles have hydrophobic properties.
It has long been known that it is possible to influence the hydrophilic or hydrophobic character of inorganic or organic particles. This can be accomplished by adsorption of substances on the surface of a particle, especially by chemical reactions with reactive groups, which are at the surface of the particle. The state of this art is shown extensively in the literature.
For example, German Offenlegungsschrift No. 23 13 073 describes a process for chemically modifying the surfaces of inorganic solids having hydroxyl groups. The characteristic feature of the process is that alkyl chains, which have reactive hetero atoms, are chemically linked to these surfaces. Moreover, these reactive hetero atoms can be replaced by chemical functional groups. A typical example of such a modifying reaction is the conversion of the hydroxyl groups linked to the surface of the solid with bromomethylmethyldichlorosilane, wherein the bromine atom is being replaced in a further reaction by an SO.sub.3 H--, NH.sub.2 --, COOH-- or a polyethylene glycol radical. Materials, such as, silicon dioxide (silica gel), aluminum oxide, titanium dioxide, zirconium dioxide, and other inorganic carriers, such as, glass spheres and sand, can be used as solids. The chemically modified solids are used as the stationary phase in a separating system, for example, in chromatography, or as a catalyst. It is evident that the suitability of such modified solids as the stationary phase in chromatography can be influenced significantly by such modifications of the surface, since the physical and chemical behavior of the modified solid relative to the mobile phase is determined essentially by the properties of its surface. Due to the nature of the modifying reaction and in view of the intended use, it may be assumed that the entire surface of the particles participates in the modifying reaction. If the modifying agent is used in an amount less than equivalent, so that not all of the reactive groups at the surface of the particles can be modified, it may be assumed that the distribution of the modified groups on the surface follows statistical laws so that the modified particle behaves equally hydrophilic or hydrophobic at any position on its surface.
German Offenlegungsschrift No. 21 07 082 discloses a surface-treated, pulverized silicon dioxide with a surface area of at least 10 m.sup.2 /g, which is characterized by the fact that it has a degree of hydrophobicity of 5 to 35, as measured by the methanol titration test. The methanol titration test, also referred to as the methanol number, indicates a frequently used specific quantity for determining the degree of hydrophobicity. The test is carried out by adding 0.2 g of the powder to be tested to 50 ml water in a 250 ml conical flask. If the powder has a hydrophobic surface, it is not wetted by the water and floats on the surface of the water. Then, methanol is slowly added from a burette until all of the powder to be tested has been wetted. The end point of the methanol addition is reached when all of the powder is suspended in the liquid. The methanol number is the percentage of methanol in the liquid methanol/water mixture, when the end point is reached.
In German Offenlegungsschrift No. 21 07 082, the silicon dioxide powder is hydrophobized by treating the surface with a silane, containing groups which react with the hydroxyl groups of the silica, as well as the hydrophobic groups. As a result of the reaction of the hydroxyl groups at the surface of the silicon dioxide with the reactive groups of the silane, the particles are covered with a coating in which the hydrophobic groups of the silane are extended toward the outside. An example of such a reactive siloxane is methyltriethoxysilane. A gradation of the hydrophobicity of such particles can be brought about by using organosilicon compounds for the modification, which also have hydrophilic groups, for example, oxyalkylene groups. This Offenlegungsschrift also mentions that such hydrophilic groups are distributed at random within the hydrophobic groups, that is, that the coating of the surface proceeds according to statistical laws so that the probability of encountering a hydrophilic or a hydrophobic group on the surface of a modified particle is always the same, depending on the concentration of these groups. According to the teachings of this Offenlegungsschrift, the surface-treated silicon dioxide increases the compatibility of silicon dioxide with fire extinguishing agents.
German Offenlegungsschrift No. 31 32 370 describes a process for the tertiary recovery of oil by means of surfactant flooding, in which an effective amount of a surface-modified silicon dioxide is added to the surfactant-containing aqueous liquid. The silicon dioxide is modified preferably with an .alpha.,.omega.-polysiloxanediol. Moreover, the silica has a particle size of 0.005 .mu.m to 2 .mu.m. It is added in an amount of 10 to 1,000 ppm to the surfactant solution. This Offenlegungsschrift states that the "silicon dioxide/dialkylsiloxane compound" is highly surface active because it consists of a hydrophobic and a hydrophilic portion. Since only the surface reaction with polysiloxanediol is described, it must be assumed that only the alleged hydrophilic portion in the remaining unmodified hydroxyl groups of the finely divided silicon dioxide is observed. German Offenlegungsschrift No. 31 32 370 does not describe how the silicon dioxide particles react with the polysiloxanediol. In accordance with the state of the art, it must be assumed that the siloxanediol is allowed to react with the surface of the silicon dioxide particles, resulting presumably in a condensation reaction which proceeds uniformly at all regions of the particle.
German Offenlegungsschrift No. 29 46 727 relates to a product used to separate water-oil mixtures. This product is prepared by reacting an inorganic solid having surface hydroxyl groups, with a silane having groups that react with the hydroxyl groups of the solid, such as, for example, halogen, alkoxy groups, or a halogenated hydrocarbon radical, which then, in a second step of the process, enters into a reaction with an N-substituted imide of an alkenyl-substituted dicarboxylic acid. After this modifying reaction, there is at the surface of the solid particle, a radical of relatively high molecular weight, which has hydrophilic NH-groups as well as hydrophobic alkenyl groups. The inorganic solid may be silicon dioxide, aluminum dioxide, titanium dioxide, zirconium dioxide, glass sepiolite or a zeolithic molecular sieve. If a filter bed is formed from particles so modified, and water contaminated with oil, or oil contaminated with water, is passed through this bed, the contamination is removed from the carrier phase.
Furthermore, German Patent No. 27 43 682, which is part of the state of the art, relates to the treatment of oxide or silicate particles with special organosilanes. These organosilanes have a substituent of oxyalkylene groups as well as hydrolyzable groups, which are intended to effect the linkage of the silane to the solid particles. The particles, so modified, show increased dispersebility and are intended to be used as fillers in heat-curable polyester compositions.
A common feature of all of these modified particles of the state of the art is that the hydrophilicity or hydrophobicity of the surface is influenced selectively in order to endow the particles with certain properties desirable for their technological application. These application-related technological properties are based especially on better compatibility and wettability of the particles with or by the medium containing the particles. The solids with modified surface properties known from the state of the art, have hydrophobic or hydrophilic groups, which are located at the surface of the particles in a uniform random distribution. Therefore, is is only possible to hydrophobize a hydrophilic particle more or less pronouncedly. This is shown by the distribution of the particles between two immiscible liquids.
If hexane, a nonpolar liquid, is placed over a layer of water, a polar liquid, and pyrogenic silica whose surface has not been treated, is added, then the untreated pyrogenic silica is distributed almost exclusively in the water because of the presence of hydroxyl groups on the surface of the silica. If the silica is hydrophobized according to one of the processes known from the state of the art so that practically all of the hydroxyl groups are modified and any remaining hydroxyl groups are sterically shielded, so that their contribution to surface properties can be disregarded, the silica so hydrophobized is distributed almost exclusively in the hexane phase. However, if the pyrogenic silica is only partially hydrophobized to varying extents, and the degree of hydrophobicity determined by the methanol number, then the distributions obtained between the two phases correlate with the degree of hydrophobization.